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What is a CSO?

About one-third of Portland has a sewer system that carries both sewage from homes and businesses and stormwater runoff from streets in the same pipes. On both dry and rainy days, the combined sewer system carries wastewater to the city’s Columbia Boulevard Wastewater Treatment Plant in north Portland.

Large tunnels on the south side of the Columbia Slough and on both sides of the Willamette River collect combined sewage during rainy weather and convey it to the treatment plant. If the tunnel system fills up during very heavy rainstorms, some combined sewage can overflow to the river or slough. These are called combined sewer overflow (CSO) events.

Why did the city build a combined sewer system?

The city constructed most combined sewer pipes between 80 and 120 years ago. They once carried sewage, industrial wastewater and street runoff directly to the Willamette River and Columbia Slough with no treatment.

When Portland opened its first wastewater treatment plant in 1952, new pipelines intercepted sewage and carried it all to the treatment plant during dry weather. Even though combined sewers overflowed during rainy weather, having a treatment plant improved water quality in the slough and river. As time went on and Portland grew, more streets and other hard surfaces created more stormwater runoff and combined sewer volume increased.

According to the U.S. Environmental Protection Agency (EPA), more than 800 cities in the United States have combined sewer systems. Many of them are still designing or constructing required programs to control combined sewer overflows. Portland completed its CSO control program in 2011.

From 1990 to 2011, the city built projects to keep stormwater from flowing to the combined sewer system, large tunnels to capture combined sewage during wet weather, and pumps to send it to the treatment plant. The city also added treatment plant facilities to handle all the extra water.

This CSO control program cost Portland sewer ratepayers $1.4 billion. It significantly improved water quality in the Willamette River and Columbia Slough, making them safe for people and healthier for fish and wildlife.